RigTonightIs tonight good for astrophotography? The six things to check
A clear sky is only the start. Plenty of cloudless nights still give washed-out, blurry or dewy images. Before you carry the gear outside, check these six things, roughly in order of how much they matter.
1. Cloud, at every height
Cloud stops everything, so it matters most. Look at three layers:
- Low cloud (fog, stratus, cumulus) blocks the sky completely.
- Mid cloud usually does too.
- High cloud (thin cirrus) can be hard to see by eye at night but dims and scatters starlight, adding gradients and halos. A "clear" forecast with thin cirrus is a poor night for galaxies.
Forecasts disagree, especially about thin and broken cloud. Comparing several weather models tells you how confident to be: if ECMWF, GFS, ICON and others all say clear, believe it. See Which weather model should astronomers trust?.
2. Is it actually dark?
Sunset isn't darkness. The sky keeps brightening the background until the sun is 18° below the horizon, the start of astronomical darkness. In summer at higher latitudes that may not happen at all. When is it dark enough? explains the three twilights.
3. The moon
A bright moon raises the sky background across the whole sky, worst near the moon itself. Galaxies, reflection nebulae and faint dust suffer most; emission nebulae through narrowband filters suffer least. Check the phase, when it rises and sets, and how far it is from your target. See Astrophotography and the moon.
4. Transparency
Transparency is how clear the air is: how much haze, smoke, dust and humidity sit between you and space. On a transparent night faint stars are crisp right down to the horizon; on a poor one the sky looks milky even with no cloud. Poor transparency cuts contrast on faint targets, especially broadband ones. Aerosol forecasts (dust, smoke, pollution) and very high humidity are the clues.
5. Seeing
Seeing is how steady the air is. Turbulence, often from fast jet-stream winds high overhead, smears stars into bigger blobs. You notice it as twinkling to the eye and fat stars in your images.
- At short focal lengths (wide fields), seeing barely matters, because a pixel already covers more sky than the blur.
- At long focal lengths (SCTs, galaxies, planetary nebulae), seeing decides how much detail you get.
6. Dew, wind and temperature
- Humidity close to 100% means dew. Bring heaters. See How to stop dew.
- Wind shakes the rig, especially long tubes and light mounts. Gusts above about 15–20 km/h make long-focal-length imaging hard.
- Cold helps cooled cameras but drains batteries faster.
Putting it together
A great night has clear skies at every height, a few hours of full darkness, the moon down or far from your target, good transparency, and calm air. A good enough night is more forgiving than that: a moonlit night with steady skies is perfect for narrowband nebulae, and a slightly hazy night is fine for bright clusters.
RigTonight's Tonight tab checks all six for your location, hour by hour. It compares cloud from five weather models, adds darkness, moon, estimated seeing and transparency and your light pollution, and gives each hour one score. Then it ranks the targets worth shooting with your rig in the best window.